<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>


<script>
    var A = {
        n: 4399
    };
    var B = function(){
        this.n = 9999
    };
    var C = function(){
        var n = 8888
    };
    B.prototype = A;
    C.prototype = A;
    var b = new B();
    var c = new C();
    A.n++
    console.log(b.n); //? b里已经有n属性了 9999
    console.log(c.n); //? c里没有n属性 只有往原型上找咯 4400
</script>
</body>
</html>
